Magneto-inductive flow measuring devices utilize the principle of electrodynamic induction for measuring volumetric flow, e.g. flow rate: Charge carriers of the medium moved perpendicularly to a magnetic field induce a voltage in electrodes arranged essentially perpendicular to the flow direction of the medium and perpendicular to the direction of the magnetic field. The measurement voltage induced in the electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube. Thus, the induced voltage is proportional to the volume flow rate. If the density of the medium is known, the mass flow in the pipeline or the measuring tube can be determined. The measurement voltage is usually sensed via the electrode pair, which is arranged in the region of the measuring tube in which the maximum magnetic field strength and thus the maximum measurement voltage is to be expected. The electrodes are usually galvanically coupled with the medium; however magneto-inductive flow measuring devices with contactless, capacitively coupled electrodes are also known.
The measuring tube can be manufactured either from an electrically conductive material, e.g. stainless steel, or from an electrically insulating material. If the measuring tube is manufactured from an electrically conductive material, then the region coming in contact with the medium must be lined with a liner of an electrically insulating material. The liner is composed, for example, of a thermoplastic, a thermosetting plastic or an elastomeric synthetic material, depending on temperature and medium. However, magneto-inductive flow measuring devices with a ceramic lining are also known.
If no voltage supply is available, field devices are often battery operated. This enables an autarkic use of the corresponding field devices. Magneto-inductive flow measuring devices especially can be operated in this way maintenance free for a number of years.
With the application of measuring electrodes contacting the medium, there form, at the interface between the metal measuring electrode and the medium flowing through the measuring tube, galvanic elements, which result in an electrochemical disturbance potential. This electrochemical disturbance potential is variable and dependent on different changing environmental conditions such as temperature, pressure, composition of the medium, material of the measuring electrodes and material of the measuring tube. Thus for example, the composition of the surface of the measuring electrodes can change as a result of the forming of a passivating layer or as a result of corrosion. The varying electrochemical disturbance potential, which is proportional to the flow velocity of the medium flowing through the measuring tube, is superimposed on the actual measurement voltage. It goes without saying that an electrochemical potential changing over time negatively influences the accuracy of measurement of a conventional magneto-inductive flow measuring device. Methods have become known which eliminate these disturbance signals. It is especially critical, when the medium to be measured is a medium with a small conductivity that flows through the measuring tube with a relatively high flow velocity. Due to the influence of the relatively large disturbance voltage on the measurement voltage, the danger is, then, that the measurement voltage disappears in the noise and a reliable and repeatable flow measurement becomes impossible.
The flow proportional measurement voltage of the electrodes amounts to a few mV, possibly only a few μN in the case of very low flow velocity. The electrochemical disturbing voltages, which occur at the interface between electrode and liquid, are superimposed on the flow proportional measurement voltage, as already mentioned. The electrochemical disturbance voltage is a direct voltage, which is applied between the measuring electrodes. This disturbance direct voltage can be more than 100 mV and is some orders of magnitude greater than the flow proportional measurement voltage to be evaluated. Additionally, grid frequency disturbing voltages are superimposed frequently on the measurement voltage.
In order to differentiate the disturbance direct voltage from the signal voltage, targeted time dependent modulation of the magnetic field, thus the electrical current which flows through the coils of the magneto-inductive flow measuring device exciting the magnetic field, is necessary.
At a magnetic field strength B=0, the measurement voltage U is equal to zero. If one increases the electrical current through the coils, the strength of the magnetic field B rises and correspondingly the measurement voltage U also rises. In the case of a reversal of the coil current, thus an reversed magnetic field B, U likewise changes sign. This effect is applied, in order to separate the measurement voltage from the electrochemical disturbance, direct voltage.
Of all the components of a magneto-inductive flow measuring device, the coil arrangement for producing the magnetic field B passing through the measured material has the largest energy requirement.